This invention relates to a pressure control unit with two pressure control valves arranged in a housing for a vehicle hydraulic brake system. Each pressure control valve is assigned to a separate brake circuit and connected to a pressure generator on the inlet side and to a wheel cylinder on the outlet side. The pressure control valves are arranged essentially in parallel with each other and each includes a control piston subjected to a common control force by the insertion of a means arranged in the housing for the distribution of the force.
German Pat. No. DE-OS 2,748,699 describes a dual pressure regulating valve for a vehicle brake system including a pair of valves each of which includes a housing with an internal bore extending essentially in parallel with the internal bore of the other valve. Each valve includes an inlet port connected to a master brake cylinder or its equivalent, and an outlet port connected to an associated brake actuating member. Each internal bore accommodates a piston which is slidable by pressure against a preload and adapted to close the valve when a pressure which is dependent upon the magnitude of the respective preload is exceeded. Further, both pistons have a common preload generator with a preload distributor distributing the generated preload to the pistons at an essentially constant raio under normal operating conditions. The preload distributor includes a beam which has a piston at each of its ends and between its ends is operatively connected to the common preload generator. The beam is tiltable within limits so that in the event of a failure of part of the brake system an increased portion of the preload of the common preload generator will act on the valve of the common preload generator will act on the valve of the intact brake circuit.
In the known pressure regulating valve, the tipping lever arrangement requires the provision of a guide and a torsion-preventing device in order to maintain the operability of the arrangement. This renders machining and assembly of the pressure control unit more difficult. Another disadvantage is that in the event of failure of one circuit the pressure limitation or pressure reduction will, nevertheless, occur in the intact circuit even if the valve's change-over point is at a higher pressure. In the event of failure of a brake circuit, it is, however, essential to achieve the best possible braking effect with the intake brake circuit. Therefore, any reduction of the braking effect or a limitation of the brake pressure is far from being desirable in that event.